1. Field of the Invention
The present invention relates to a semiconductor device such as a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
2. Description of the Related Art
A power semiconductor device, typically a power MOSFET, comprises a semiconductor chip structured such that a plurality of cells having commonly connected gates are formed in an epitaxial layer (semiconductor region) disposed on a semiconductor substrate. The power MOSFET, having a low on-resistance and a possibility of fast switching, can control a high-frequency large current efficiently. Accordingly, the power MOSFET has been used widely as a small power converter such as a component of a power source in a personal computer.
The power MOSFET includes a semiconductor region connecting a source region to a drain region, which is generally referred to as a drift region. When the power MOSFET is turned on, the drift region serves as a current path. When it is turned off, a depletion layer extends from a pn junction formed between the drift region and a base region to retain the breakdown voltage of the power MOSFET.
The on-resistance of the power MOSFET greatly depends on the electric resistance of the drift region. Therefore, lowering the on-resistance may require an increase in the impurity concentration in the drift region to lower the electrical resistance of the drift region. An increased impurity concentration in the drift region, however, makes the extension of the depletion layer insufficient and lowers the breakdown voltage. Thus, in the power MOSFET, lowering the on-resistance and elevating the breakdown voltage are related to have a tradeoff therebetween.
To solve the problem, there has been proposed a power MOSFET, which includes a drift region having a superjunction structure (JP-A 2004-134714, Paragraphs 0043-0051, FIGS. 15-17). The superjunction structure is such a structure that includes p-type semiconductor regions in the form of pillars and n-type semiconductor pillar regions in the form of pillars ,periodically arranged in a direction parallel to the surface of a semiconductor substrate. Depletion layers extend from pn junctions formed between these semiconductor regions to retain the breakdown voltage. Accordingly, even if increasing the impurity concentration to lower the on-resistance reduces the extension of the depletion layer, reducing the widths of these semiconductor regions makes it possible to completely deplete these semiconductor regions. Therefore, the superjunction structure is capable of lowering the on-resistance and elevating the breakdown voltage of the power MOSFET at the same time.
For further elevating the breakdown voltage, there has been proposed another power MOSFET, in which the impurity in a p-type semiconductor region and the impurity in an n-type semiconductor region are balanced, that is, charge-balanced (JP-A 2004-119611, Paragraphs 0062-0065, FIG. 7).